Star polymers of dienes, vinylarenes and alkyl methacrylates as modiied viscosity index improvers

ABSTRACT

Star polymers having first polymeric arms comprising a hydrogenated conjugated diene and smaller second polymeric arms of a methacrylate resist coupling of methacrylate blocks attached to different cores and have improved performance as viscosity index improvers. The polymerized methacrylate units are converted to amide or imide groups by reaction with a primary or secondary amine to give the viscosity index improver dispersant properties.

FIELD OF THE INVENTION

This invention relates to star polymers having ester groups. Moreparticularly, the invention relates to modified star polymers ofhydrogenated isoprene useful as viscosity index improvers for oilcompositions.

BACKGROUND OF THE INVENTION

Star polymers useful as viscosity index improvers for oils andlubricants are described in U.S. patent application Ser. No. 942,019filed Sep. 8, 1992 (T4773N). The star polymers have blocks of amethacrylate at the end of polymeric arms that are grown from a coupledcore of a smaller star polymer. The smaller star polymers are preparedby crosslinking small blocks of styrene, isoprene, or butadiene withdivinyl benzene. The larger polymeric arms are grown at active lithiumsites on the central core of the small star polymers. The methacrylateblocks at the end of the polymeric arms have a strong tendency to coupletwo or more of the star polymer cores. Coupling of the star polymercores effectively broadens the molecular weight distribution of the starpolymers and reduces performance as viscosity index improvers forlubricating oils. Elimination of the coupling would improve performancein lubricating oils.

Addition of amide groups to polymeric viscosity index improversincreases the dispersency of sludges in lubricating oils. Conversion ofacrylic groups in acrylic polymers to amide groups is described in U.S.Pat. No. 4,246,374 which teaches reaction of the acrylic group with ananhydrous primary amine. The conversion reaction occurs between outerlimits of about 200° C. and 450° C.

Conversion of the acrylic groups to amide groups will occur in thepresence of other monomer units such as styrene, butadiene, or isoprene.However, butadiene and isoprene units in the polymers thermally degradeat temperatures between about 200° C. and 450° C. Conversion of acrylicgroups to amide or imide groups at lower temperatures would reducedegradation of polymers containing butadiene or isoprene.

SUMMARY OF THE INVENTION

Star polymers having polymeric arms of a hydrogenated conjugated dieneand substantially smaller polymeric arms of a methacrylate resistcoupling of methacrylate blocks attached to different cores resulting inimproved performance as viscosity index improvers. The methacrylateblocks are easily converted to amide or imide groups by reaction with aprimary or secondary amine to give the viscosity index improverdispersant properties. Conversion of polymerized t-butyl methacrylate toamide or imide groups occurs between about 180° C.-400° C.

DESCRIPTION OF THE INVENTION

The present invention includes a polymer molecule comprising at least 3first arms comprising a preponderance of a hydrogenated, polymerizedconjugated alkadiene, each arm having a molecular weight from 10,000 to200,000, at least 3 arms comprising a preponderance of a polymerizedmethacrylate, each arm having a molecular weight from 1,000 to 50,000,and a central core comprising a polymerized bisunsaturated monomer,wherein the central core connects the arms in a star configuration andthe first arms are longer than the second arms. Preferably, at least 80%of the polymerized methacrylate units have been converted to amidegroups.

The star block polymers of the invention are produced by preparing firstarms comprising a preponderance of the conjugated diene, coupling thefirst arms by polymerizing the bisunsaturated monomer, growing secondarms comprising the alkyl methacrylate from the polymerizedbiunsaturated monomer, and hydrogenating the polymerized conjugateddiene. The conjugated diene, preferably isoprene or butadiene, isanionically polymerized with an initiator, preferably an alkyl lithium.Alkyl lithium initiators having a secondary alkyl group are preferred.Most preferred is sec-butyllithium.

The bisunsaturated monomer couples the conjugated diene arms into a"star" molecule having a plurality of the first polymeric arms radiatingfrom a central core which comprises the polymerized bisunsaturatedcompound. After coupling, the core of the molecules contain residuallithium sites which initiate the growth of the second polymeric arms.

One or more of the first polymeric arms may comprise a polymerizedvinylarene in a random, tapered, or block configuration with thepolymerized conjugated diene. The preferred vinylarene is styrene andthe preferred styrene content for the star polymers is less than 10%.

The polymerization to produce the first polymeric arms is conducted bythe conventional method of contacting the monomer and polymerizationinitiator in a suitable reaction solvent under moderate reactionconditions. Hydrocarbon reaction solvents, particularly cycloaliphatichydrocarbon solvents such as cyclohexane are suitable as reactionsolvents. It is useful on some occasions to employ a reaction solvent ofgreater polarity and in such instances a mixed solvent, often a mixtureof cyclohexane and a polar co-solvent, e.g., an ether co-solvent such asdiethyl ether or tetrahydrofuran, is used. The use of cyclohexane orcyclohexane-diethyl ether as reaction solvent is preferred. Thepolymerization temperature is moderate, for example from about 10° C. toabout 80° C. and it is often useful to conduct this polymerization atambient temperature. The reaction pressure is a pressure sufficient tomaintain the reaction mixture in a liquid phase. Typical reactionpressures are from about 0.8 atmospheres to about 5 atmospheres.

Control of the molecular weight of the first polymeric arms is achievedby conventional methods such as controlling the ratio of initiator tomonomer. The polymeric arms are conventionally termed a living polymerbecause of the presence therein of an organometallic site. The firstpolymeric arms preferably have a peak molecular between 10,000 and200,000, most preferably between 20,000 and 100,000.

The first polymeric arms serve as the polymerization initiator for thebisunsaturated monomer which crosslinks to form the central core of thestar polymer molecules. A variety of bisunsaturated monomers are usefulin the production of the core of the star block polymers of theinvention. Preferred bisunsaturated monomers are di(alkenyl) aromaticcompounds having up to 20 carbon atoms and up to 2 aromatic rings,including divinylbenzene, divinyltoluene, divinylbiphenyl,divinylnaphthalene, diisopropenylbenzene, diisopropenylbiphenyl anddiisobutenylbenzene. Most preferred is divinylbenzene.

The crosslinking of the bisunsaturated monomer with the first polymericarms is preferably conducted by adding the bisunsaturated monomer to thereaction mixture containing the first polymeric arms. The use of thesame or similar reaction conditions and reaction solvent are suitablefor the crosslinking reaction to form the core of the star blockpolymer.

The core of crosslinked bisunsaturated monomer has a plurality oforganometallic sites which serve as the polymerization initiator for themethacrylate which forms the relatively smaller second polymeric arms.Alkyl methacrylates are preferred. The alkyl group on the alkylmethacrylate monomer has up to 30 carbon atoms, preferably up to 20carbons. The alkyl methacrylate is polymerized through the ethylenicunsaturation of the methacrylate group. The alkyl methacrylate monomerswhich are polymerized according to this invention include methylmethacrylate, ethyl methacrylate, sec-butyl methacrylate, t-butylmethacrylate, sec-amyl methacrylate, octyl methacrylate, decylmethacrylate, dodecyl methacrylate and octadecyl methacrylate.Polymerization is preferably conducted in the reaction mixturecontaining the star molecules having organometallic sites on the centralcore.

The choice of alkyl methacrylate will in part depend upon the particularnature of the star block polymer desired. However, the production ofpolymerized alkyl methacrylate branches wherein the alkyl is primary andof few carbon atoms is relatively difficult because of the rather lowreaction temperatures that are required to produce the polymerized alkylmethacrylate branches. Alternatively, the production of polymerizedalkyl methacrylate branches wherein the alkyl moiety is a higher alkylmoiety is also difficult because of the relatively inactive character ofsuch alkyl methacrylates and the difficulty of readily obtaining thedesired alkyl methacrylate monomer. The preferred alkyl methacrylatesfor forming the star block polymer of methacrylate-containing branchesis a branched-butyl methacrylate, i.e., sec-butyl methacrylate ort-butyl methacrylate. The star block polymers resulting from use ofthese methacrylates are preferred products because of the desirableproperties thereof and because of the relative ease of production. Starblock polymers incorporating other alkyl methacrylate moieties areproduced directly from the corresponding alkyl methacrylate but it isoften desirable to produce such polymers by initially employing abranched-butyl methacrylate to produce a star block polymer havingbranched-butyl methacrylate branches and subsequently trans-esterifyingthe initial star block polymer product to incorporate the desired alkylmoieties.

In the production of a branched-butyl methacrylate-containing polymersuitable reaction conditions typically include a reaction temperaturefrom about -80° C. to about 80° C. with the lower portion of that rangebeing preferred for polymerization of sec-butyl methacrylate and thehigher portion of the range being preferred for t-butyl methacrylate.The polymerization pressure is suitably sufficient to maintain thereaction mixture in a liquid phase, typically up to about 5 atmospheres.

The star polymers are hydrogenated to reduce the extent of unsaturationin the aliphatic portion of the polymer. A number of catalysts,particularly transition metal catalysts, are capable of hydrogenatingthe aliphatic unsaturation of the star polymers. It is preferred toemploy a "homogeneous" catalyst formed from a soluble nickel compoundand a trialkylaluminum. Nickel naphthenate or nickel octoate is apreferred nickel salt. Although this catalyst system is one of thecatalysts conventionally employed for selective hydrogenation in thepresence of aromatic groups, other "conventional" catalysts are notsuitable for hydrogenation of the conjugated alkadienes in the estercontaining polymers.

In the hydrogenation process, the base polymer is reacted in situ, or ifisolated is dissolved in a suitable solvent such as cyclohexane or acyclohexane-ether mixture and the resulting solution is contacted withhydrogen gas in the presence of the homogeneous nickel catalyst.Hydrogenation takes place at temperatures from about 25° C. to about150° C. and hydrogen pressures from about 15 psig to about 1000 psig.Hydrogenation is considered to be complete when at least about 90%,preferably at least 98%, of the carbon-carbon unsaturation of thealiphatic portion of the base polymer has been saturated, as can bedetermined by nuclear magnetic resonance spectroscopy.

The hydrogenated star polymer is then recovered by conventionalprocedures such as washing with aqueous acid to remove catalystresidues, solvent removal, or addition of a non-solvent to coagulate thepolymer. A typical non-solvent for this purpose is aqueous methanol.

DESCRIPTION OF PREFERRED EMBODIMENT

The preferred polymers of the invention comprise an average per moleculeof 10-50 first arms consisting of hydrogenated, polymerized isoprene orblocks of styrene and hydrogenated, polymerized isoprene, the first armshaving a peak molecular weight from 10,000 to 100,000, at least 10-50second arms consisting of polymerized t-butylmethacrylate, the secondarms having a peak molecular weight from 1,000 to 10,000, wherein atleast 80% of the polymerized t-butylmethacrylate units have beenconverted to amide or imide groups, and one central core per molecule,the core comprising polymerized divinylbenzene, wherein the centralcores connect the first and second polymeric arms in a starconfiguration.

The molecular weight of the star polymers of the invention will varywith the choice of reaction conditions, reaction solvent and therelative proportions of monomeric reactants as well as determined inpart by whether the functionalized branches are homopolymeric or containan internal portion of polymerized anionically polymerizable monomer.The star polymers of particular interest have a peak molecular weightfrom about 33,000 to about 5.5×10⁶ and most preferably from about100,000 to about 3×10⁶. The precise peak molecular weight will vary frommolecule to molecule and the above values are average values. It is,however, characteristic of the star polymers of the invention that thepolymer has a rather narrow molecular weight distribution.

The star polymers are represented by the formula

    (A--).sub.t --C--[--M].sub.s                               (I)

wherein C comprises the crosslinked bisunsaturated monomer, A comprisesthe hydrogenated, polymerized conjugated diene, M comprises thepolymerized alkyl methacrylate, wherein each alkyl independently has upto 30 carbon atoms polymerized through the ethylenic unsaturation of themethacrylate moiety, s is the number of polymeric arms grown from theblock of crosslinked unsaturated monomer, and t is the number ofconjugated alkadiene arms up to 50 which is equal to or greater than s.

While the proportions of the moieties represented by the terms C, A, andM will vary somewhat from molecule to molecule, the percentage of themolecular weight of the molecule attributable to the central core, C, isno more than about 10% and preferably no more than about 2%.

Each A block or segment in the preferred star polymer preferablycomprises at least 90% by weight of the hydrogenated, polymerizedconjugated diene. The conjugated alkadienes preferably have up to 8carbon atoms. Illustrative of such conjugated alkadienes are1,3-butadiene (butadiene), 2-methyl-1,3-butadiene (isoprene),1,3-pentadiene (piperylene), 1,3-octadiene, and 2-methyl-1,3-pentadiene.Preferred conjugated alkadienes are butadiene and isoprene, particularlyisoprene. Within the preferred polyalkadiene blocks or segments, thepercentage of units produced by 1,4 polymerization is at least about 5%and preferably at least about 20%.

Each M is preferably a methacrylate block or segment comprising at least90% by weight of a polymerized alkyl methacrylate. Homopolymeric Msegments or blocks of alkyl methacrylates are most preferred. The alkylmethacrylates have the structure: ##STR1## wherein R is an alkyl groupcomprising from 1 to 30 carbon atoms, preferably 1 to 10 carbon atoms.The most preferred alkyl methacrylates are s-butyl methacrylate andt-butyl methacrylate. The t-butylmethacrylate monomer is commerciallyavailable in high purity from Mitsubishi-Rayon, Japan.

Less pure t-butylmethacrylate is available from Monomer, Polymer andDajac and can be used if passed through a column of alumina and 13Xzeolite to remove methacrylic acid and t-butylalcohol. The preferredzeolites have a cavity size no less than 10 angstroms such as Zeolite13X which has the formula Na₈₆ (AlO₂)₈₆ (SiO₂)₁₀₆.267H₂ O.

The star polymers of this invention have the advantage of little or nocoupling of two or more molecules during polymerization of the alkylmethacrylate.

The amide or imide groups in the preferred polymers of the invention areproduced by heating the base polymers to a temperature from about 180°C.-400° C. in the presence of a primary or secondary amine. Heating ispreferably conducted in an extruder having a devolatization section toremove volatile by-products.

Primary amines useful for the invention include compounds having thestructure R-NH₂ as described in column 3, lines 32-62, of U.S. Pat. No.4,246,374 which disclosure is incorporated by reference herein. The mostpreferred primary amine is N,N-diethylaminopropylamine. Secondary aminesof the type R₁ R₂ NH will result in useful amide groups, but are lesspreferred.

The polymers of the invention, like the base copolymers, contain polargroups and have utilities conventional for such polymers. The polarpolymers are particularly useful as both the dispersant and viscosityindex improver in motor oils.

The invention is further illustrated by the following IllustrativeEmbodiments which should not be construed as limiting.

EXAMPLE 1

A first reactor was charged with 270 pounds of cyclohexane and 30 poundsof styrene monomer. To the stirred mixture 6.5 pounds ofsec-butyllithium was added and the styrene was polymerized for 10half-lives at 60° C.

In a second reactor 273 pounds of cyclohexane and 50 pounds of isoprenemonomer were titrated with sec-butyllithium to remove any impurities.Then 27 pounds of the living homopolystyrene from step 1 was added tothe isoprene and the isoprene was polymerized for 12 half-lives at 60°C. Next, 200 ml of divinylbenzene (55% dvb) was added to the livingstyrene-isoprene polymeric arms and reacted at 80° C. for 30 minutes toform the living star polymer.

The temperature of the star polymer mixture was lowered to 35° C. and1.20 pounds of tert-butylmethacrylate (tBMA) was added to the reaction.The tBMA was polymerized for 30 minutes at 35° C. to form the secondpolymeric arms, and the reaction was quenched with 19 ml of methanol.

The star polymer of Example 1 was hydrogenated using a catalyst composedof nickel octoate reduced by triethyl aluminum. The ratio of nickel toaluminum for this particular example was 1:2.3. The total catalystcharge was periodically increased to give a product with low residualunsaturation.

EXAMPLE 2

A reactor was charged with 12,300 grams of dry cyclohexane and 1,360grams of isoprene monomer. The cyclohexane and isoprene were titratedwith sec-butyllithium to remove impurities, then 26.8 ml of 1.45Msec-butyllithium was added to polymerize the isoprene. The isoprene wasallowed to react for ten half-lives at about 60° C. Then 32 ml of 55%divinylbenzene was added to couple the star polymer.

A first two liter Buchi reactor was charged with 1110 grams of theliving star polymer solution. The stirred solution was reacted with 5.7g of tBMA monomer, dissolved into 21.5 ml of cyclohexane, for 1 hour.The reaction was then quenched with 0.4 ml of methanol.

A second two liter Buchi reactor was charged with 1035 grams of theliving polymer solution. The stirred solution was reacted with 5.3 gramsof tBMA monomer, dissolved into 20 ml of cyclohexane, for 1 hour. Thereaction was quenched with 0.3 ml of methanol.

The polymer solutions from both Buchi reactors were then combined andhydrogenated with the nickel catalyst from Example 1 to remove theunsaturation in the polyisoprene blocks.

EXAMPLE 3 Conversion of TBMA to Amide

The conversion of the polymerized t-butylmethacrylate in Examples 1 and2 to amide groups was carried out in an extruder. For this particularexample a Brabender melt mixer was used. The Brabender was heated to250° C. and 60 grams of polymer was added with the mixing blades at 100rpm. When the melt was uniformly mixed, a mixture ofN,N-diethylaminopropylamine (DAP) and Penrico Oil was added over 3minutes time (the oil serves to prevent the DAP from vaporizing out ofthe Brabender before it can mix with the polymer melt). The sample wasallowed to mix for 3 minutes longer and then was removed from theBrabender. FT-IR analysis shows conversion to the amide, the ester peakat 1726 cm⁻¹ is replaced with an amide peak at 1667 cm⁻¹. Analysis byFT-IR revealed that the conversion to amide was at least 80% based onthe ratio of carbonyl absorbance to amide carbonyl absorbance.

Comparative Example A

A 600 ml beaker, contained inside a glove box with a dry nitrogenatmosphere, was charged with 380 ml of cyclohexane and 20 ml ofdiethylether. To this was added 3 ml of 1.4M sec-butyllithium followedby 10 ml of styrene monomer. The styrene monomer was polymerized for 20minutes at ambient temperature. Next, 1 ml of 55% active divinylbenzenewas added to the reaction and allowed to couple for 15 minutes. Finally,50 ml of lauryl methacrylate monomer, treated with 10 microliters oftriethyl aluminum, was added to the reaction as a steady stream andallowed to react for 30 minutes. The reaction was quenched with methanoland the polymer was recovered by precipitation in methanol. GPC analysisof the product showed a broadly distributed, polymodal material.

EXAMPLE 4 Dispersant Viscosity Index Improver

The star block copolymer from Example 3 having no styrene was blendedwith a motor oil to give SAE 5W-30 and 10W-40 formulations. The oils inTable 3 were blended in Exxon 100N LP, the DI package was composed of anexperimental Lubrizol additive, Acryloid 155 pour point depressant wasadded at 0.5 wt % and the polymer concentrate to give about 11 cStkinematic viscosity at 100° C. The cold cranking simulator (CCS)viscosity was then measured at -25° C. For Comparison B, a star polymerprepared as in Example 2, but having no tBMA was blended with the sameoil to give oil formulations having the same kinematic viscosity (andsame CCS viscosity for 10W-40 oils).

Each of the polymers was then blended in Exxon 100N LP at 5%concentration for further work. The blending was done at 120° C. to 130°C. using the Silverson mixer until the polymer was completely dissolved.A small amount of antioxidant was used in each case to protect thepolymer and the base oil from thermal oxidation.

Tables 1, 2, and 3 show the rheological performance of the inventivedispersant VI improvers of Example 4 and Comparison B. In Tables 1-3,TP1-MRV refers to Temperature Profile 1 in the Mini-Rotary Viscometerwhich is a measurement required for obtaining an SAE grading of alubricating oil. The TP1-MRV measurement must be conducted in accordancewith ASTM D4684 entitled "Standard Test Method for Determination ofYield Stress and Apparent Viscosity of Engine Oils at Low Temperature".Also in Tables 1-3, DIN, % Vis. Loss refers to standard diesel injectiontests which measure a decrease in viscosity due to shearing oflubricating oils in a diesel injection nozzle as determined by ASTMD-3945 or Coordinating European Counsel Test CEC L-14-A-88. Thelow-temperature properties of the inventive polymers is equal to orbetter than that of the non-dispersant polymers (see CCS and TP1-MRVdata). Table 3 shows that the inventive DVII's are particularly enhancedin CCS performance when compared to known commercial DVII polymers inidentical blends.

Fully formulated oils were prepared from the above polymer concentrates,Lubrizol DI packages, Acryloid pour depressants and Exxon base stocks.The oils in Tables 1 were blended with an experimental Lubrizol DIpackage, and Acryloid 155 was used at 0.5%. Exxon 100N LP was used forthe SAE 5W-30 oils. Exxon 100N LP and Exxon 325N were used for the10W-40 oils. The SAE 10W-40 oils were blended to about 14 cSt viscosityat 100° C. and the CCS viscosity shown in Table 1 at -20° C. The oils inTable 2 were all blended in Exxon 100N LP, an experimental Lubrizol DIpackage, Acryloid 155 pour point depressant at 0.5%, and the polymerconcentrates. The oils are blended to about 11 cSt viscosity at 100° C.and the CCS is measured at -25° C.

                  TABLE 1                                                         ______________________________________                                        Comparison of SAE 10W-40 Oil Formulations.                                    Property       Example 4  Comparison B                                        ______________________________________                                        KV, cSt        14.0       14.2                                                CCS, cP        3110       3136                                                TP1-MRV, cP    15,317     16,018                                              TBS, cP        3.67       3.77                                                DIN, % Vis. Loss                                                                             2.30       2.0                                                 ______________________________________                                    

                  TABLE 2                                                         ______________________________________                                        Comparison of SAE 5W-30 Oil Formulations.                                     Property      Example 4  Comparison B                                         ______________________________________                                        KV, cSt       10.8       11.02                                                CCS, cP       3056       2970                                                 TP1-MRV, cP   15,036     15,665                                               TBS, cP       3.03       3.11                                                 ______________________________________                                    

                  TABLE 3                                                         ______________________________________                                        Rheology of SAE 5W-30 oils containing DVII polymer.                           VII              KV, cSt  CCS, cP                                             ______________________________________                                        PARATONE ® 855                                                                             11.0     4142                                                ACRYLOID ® 954                                                                             10.8     3472                                                AMOCO ® 6565 10.7     3527                                                TLA ® 7200   10.8     3598                                                Comparison B     10.7     3245                                                Example 4        10.8     3056                                                ______________________________________                                    

The inventive polymers from Example 4 were tested for dispersancy in ablotterspot dispersancy test. An oil drain form a test car was used asthe test oil for the blotter spot test (the oil was used for 7,500 milesof city driving). The test oil was doped with the inventive polymer at0.5%, 1.0% and 2.0% wt. The well mixed solutions were then spotted onMillipore® filter discs of 0.45μ pore size. The inventive polymer showedimproved dispersancy when compared with the spot test for the undopedoil.

What is claimed is:
 1. A polymer comprising:an average per molecule ofat least 10 first arms consisting of completely hydrogenated isoprene orstyrene and completely hydrogenated isoprene, the first arms having apeak molecular weight from 10,000 to 100,000, wherein at least 5% of thepolymerized isoprene is produced by 1,4-polymerization; an average permolecule of at least 10 second arms consisting of polymerizedt-butylmethacrylate, the second arms having a peak molecular weight from1,000 to 10,000, wherein the first arms are longer than the second arms,the number of first arms is greater than or equal to the number ofsecond arms, and at least 80% of the polymerized t-butylmethacrylateunits have been converted to amide or imide groups; and one central coreper molecule, the core comprising a polymerized divinylbenzene, whereinthe central cores connect the first and second arms in a starconfiguration and the central core is no more than 10% of the polymermolecular weight.
 2. The polymer of claim 1, wherein the first armsconsist of isoprene and styrene.
 3. The polymer of claim 1, wherein theaverage number of first arms per polymer molecule is from 10 to 50 andthe average number of second arms per polymer molecule is from 10 to 50.4. The polymer of claim 1, wherein the peak molecular weight of thefirst arms is from 10,000 to 100,000 and the peak molecular weight ofthe second arms is from 500 to 10,000.
 5. A process for making apolymer, comprising the steps of:anionically polymerizing first armscomprising a conjugated diene, the first arms having a peak molecularweight from 10,000 to 200,000, wherein at least 5% of the polymerizedconjugated diene is produced by 1,4-polymerization; coupling the firstarms with a bisunsaturated compound to form central cores, wherein thecentral cores are no more than 10% of the polymer molecular weight;anionically polymerizing a methacrylate to grow second polymeric armsfrom the central cores, the second polymeric arms having a peakmolecular weight from 1,000 to 10,000, wherein the first arms are longerthan the second arms and the number of first arms is greater than thenumber of second arms; converting at least 80% of the polymerizedmethacrylate units to amide or imide groups; and completelyhydrogenating the polymerized conjugate diene.
 6. The process of claim5, wherein the first arms comprise styrene and isoprene and themethacrylate is t-butylmethacrylate.
 7. The process of claim 6, whereinthe styrene and the isoprene are present in homopolymer blocks.